U.S. military officials say the decision to strengthen their naval presence in the Gulf of Aden is designed to preserve options as Yemen’s security situation continues to worsen. They add that the presence of an Iranian convoy in waters near Yemen was "certainly one of the factors" that prompted the action.

The aircraft carrier USS Theodore Roosevelt and the guided missile cruiser USS Normandy joined seven other U.S. warships in the region Monday. Pentagon officials say an Iranian convoy of nine cargo ships is also in international waters in the Gulf of Aden, but that to their knowledge, the U.S. and Iranian ships have not yet seen each other or made any contact.

“What we see is a deteriorating security situation that could potentially result in a maritime threat,” said Pentagon spokesman Col. Steve Warren. “It's hard to predict the future, so what we need to have are options."

U.S. officials have raised concerns that the Iranian vessels could be trying to deliver weapons to Yemen’s Houthi rebels, who have long had ties to Iran. The White House says Iran has previously sent weapons to the Houthis, a charge Iran denies.

U.S. officials say whether the Iranian ships currently in the Gulf of Aden are carrying weapons is unclear, and the Pentagon Tuesday refused to definitely categorize them as a threat.

“They have not declared their intentions or what they're going to do,” Warren said. “But by having American sea power there, we're able to keep a very close eye on them.”

"We are going to continue to maintain freedom of navigation,” he said.

A senior military official official said that while it has been several weeks since the Yemen’s Houthi rebels last got arms from Tehran, the U.S. and its allies “have an interest in ensuring the Houthis don’t get any more weapons shipments.”

There have been concerns that the presence of U.S. and Iranian ships off the coast of Yemen - in an area already blockaded by Saudi Arabia, which late Tuesday announced the conclusion of a four-week air campaign against the Houthis - could lead to more strife.

“I don’t see an imminent conflict,” the official said, speaking on the condition of anonymity and playing down such concerns. “I don’t see an imminent confrontation.”

At the White House Tuesday, when pressed by reporters as to what specifically prompted the United States to step up its military presence off Yemen, Press Secretary Josh Earnest said the U.S. continues to be concerned about broader political instability in the region.

“You essentially have a Yemeni central government that has been rendered pretty ineffective in terms of being able to provide for security off the waters of their coast. They are having a lot of trouble with their security situation in their country,” he said.

Earnest said the specific mandate and mission of the USS Theodore Roosevelt remains to ensure the free flow of navigation and commerce in the Red Sea and Gulf of Aden. He said the U.S. continues to be mindful of Iran’s destabilizing activity in the region, including sending weapons to the Houthis.

“A specific arms shipment from the Iranians intended for the Houthis would be a pretty clear violation of a United Nations Secuirty Council embargo, no doubt about that,” said the White House spokesman. Earnest deflected questions, however, on whether the U.S. aircraft carrier had been deployed to specifically discourage or intercept such an arms shipment.

Prior to sending a convoy of cargo ships to the Gulf of Aden, Iran earlier said it had sent two other ships to the area for anti-piracy operations.

A senior Houthi official Tuesday criticized the U.S. decision to send additional ships to the area.

"The goal of the movement of American ships is to strengthen the siege imposed on Yemen and put the Yemeni people under collective punishment," Houthi politburo member Mohammed al-Bukhaiti told Reuters. "This step increases the level of their participation in this war."

The United Nations Security Council imposed an arms embargo on Houthi leaders last week.

U.S. Fifth Fleet spokesman Kevin Stephens said there are a number of steps the Navy could take if it has indications that weapons are on Iranian ships, including a simple request to come aboard.

In addition to the USS Theodore Roosevelt and the USS Normandy, the U.S. has seven other U.S. battleships in the area, including the Iwo Jima Amphibious Ready Group, which includes a complement of more than 2,000 U.S. Marines. There are also three U.S. resupply ships in the region.

Iran, meanwhile, is calling for an immediate cease-fire in Yemen, where Houthi rebels have seized territory amid the Saudi-led campaign of airstrikes backing President Abd-Rabbu Mansour Hadi.

Iranian President Hassan Rouhani said Tuesday the proposal, which calls for a halt in fighting, talks among the Yemeni parties and formation of an inclusive government, is the way to solve the country's crisis.

Saudi Arabia says it is stopping its Arab coalition airstrikes on the Houthis in Yemen, but is not easing the pressure on the rebels.

Saudi officials said Tuesday after four weeks of bombings, the Houthis no longer pose a threat to Yemeni civilians or other countries, including Saudi Arabia.

U.S. defense officials said Tuesday that while some of the Saudi airstrikes had been very effective, they would welcome a cease-fire and are closely monitoring developments.

In other developments, the World Health Organization is warning of an imminent collapse in Yemen's health-care services, due to increasing shortages of critical medicines and health materials, disruptions in power supplies and a lack of fuel for generators.

The United Nations says more than 600 people have been killed and about 100,000 people have been displaced by the conflict in Yemen.